In looking for and learning about planets beyond our solar system, there is always the question: are we special? On the one hand, per the Copernican Principle, the history of science is practically just a string of discoveries about how we’re not unique: we’re not the center of the Solar System, the Solar System isn’t the center of the universe, our galaxy is one of many, and heck, this might not even be the only universe. Planets, we’re discovering, are a dime a dozen. A dime a billion.

But in one very big way, we’re afraid we’re unique: in being alive. The question of Earth’s specialness is at the center of questions about life elsewhere in the universe. Do other planets have water? Are these planets in their habitable zones? Good atmosphere? Plate tectonics? The list of qualifications seems to go on and on, leading us to wonder if maybe at some point the Copernican Principle breaks down.

One such arena for debate has been the structure of our Solar System—as we’ve gained an understanding of other planetary systems, ours has started to look, rather than mediocre, downright unique. We don’t know if our exact situation is necessary for life, but with a sample size of 1, it’s been dismaying to see how different other planetary systems seem to be from our own. Similarly, we built our first theories of planet formation off what we saw in the Solar System. Exoplanet discoveries threw all that into question. We’ve seen hot Jupiters orbiting up close to their stars, planets around binary stars, and, in the focus of today’s paper, a prevalence of very eccentric orbits.

NASA

The nearly circular orbits in our solar system, not drawn to scale

While the planets in the Solar System show consistently low eccentricities in their orbits—their orbits are nearly circular—exoplanets have a much wider range of eccentricities, with a much higher average eccentricity overall. Theories of planetary system evolution have had to work to take this range into account. One approach has been to model system evolution with an eye on multi-planet interaction; prior research has predicted that multi-planet systems might have less eccentric orbits, thanks to planet-planet interactions. Today’s paper uses the data we have on exoplanets to test whether multiplicity—the presence of many planets in a system—could indeed dampen eccentricity.

The authors analyzed data for 403 cataloged planets that have been found using the radial velocity technique. (They focused on planets found via radial velocity because this detection method allows for relatively reliable measurement of eccentricity.) While most of these planets orbit their stars alone or with one other planet, some systems with four, five, and even six planets have been found; our own Solar System, with its eight planets, was also included. The authors found a strong negative correlation between multiplicity and eccentricity: the more planets a system had, the less eccentric their orbits were. This correlation is most visible when the mean and median eccentricity are plotted as a function of multiplicity (as shown to the right). The Solar System fits the trend nicely. Maybe we’re not so special after all.

Mean and median eccentricities

Mean and median eccentricities in exoplanet systems and the Solar System as a function of multiplicity (number of planets in the system). As the number of planets increases, eccentricity decreases. The plateau in eccentricity at low multiplicity may be due to contamination of the one-planet data with higher-multiplicity systems. The “SS” denotes solar system planets.

The deviation from the trend was found in one- and two-planet systems, some of which weren’t as eccentric as these findings would predict. These systems may have additional planets as yet undetected; they could be good targets for future searches for companions to known exoplanets.

This statistical analysis lends support to models of planetary formation in which planet-planet interactions within a system contribute to the circularizing of planets’ orbits. The authors suggest constraints for the relationship of multiplicity and orbital eccentricity that future models should take into account. And overall, this paper reassures us of our ongoing mediocrity. For a multi-planet Solar System, our planets’ low orbital eccentricities aren’t so eccentric after all.

Comments:

Enzo: 05/05/2014 04:02 CDT

There are some other reasons why our solar system doesn't seem to be common. No point repeating them here as they were posted recently as replies to another Planetary Society guest blog :
http://www.planetary.org/blogs/guest-blogs/2014/0416-the-birth-of-the-wanderers.html

Torbj??rn Larsson: 05/11/2014 02:29 CDT

Well, I'm going to repeat my comment there, try to point out why your claim isn't supported. (There was also a misunderstanding, see the original thread for that.)
The wide distributions of planets will make systems where you ask for tight constraints relatively rare. I.e. if you ask for a G star with a habitable Earth massed planet, there won't be many such. But why would you? The question of weirdness isn't about relative rarity but about unlikely outcomes, straining those constraints by being at the tail of the distributions. You are confusing one question for another.
That is exactly what this article pointed out. Similarly I pointed out that the Nice model shows that our system isn't a weird outcome, well within the bump of the likelihood distribution, given initial constraints. Initial constraints that won't apply to many systems, since 2 gas giants like Jupiter and Saturn is rare. There will be very few Grand Tacks, and multiple planet systems will have more densely packed planets. That is _good_ if your star is an M star, since that places many planets in the HZ.
Our planet isn't a superhabitable (larger planet so more productive and assured of having plate tectonics, calmer sun like with binary M stars), and there will be many more of those than our scrappy planet. E.g. habitability won't be rare. But at the same time our planet isn't weird and in a weird system.

Torbj??rn Larsson: 05/11/2014 02:33 CDT

I forgot: There is another problem with the gas giants. They send a lot more asteroids our way than if they would be ice giant sized (neptunes) instead. Again, our system isn't the best for habitability. But if you look for habitability and/or a near match, then the constraints are wider than if you look for a perfect match.

Bob Ware: 05/12/2014 11:09 CDT

To me our planetary system is not weird. Our star is one of countless many similar to it. We have a lot of planets, probably more than most systems to date but that may change sooner than later. What is the most weird to me is the variety of geological structural difference in the planets, even when similar in size. The second is the make up is incredibly different across the primary variety... rock, gas and ice and for the appropriate moons, water and ice. Some of the moons are (my definition) planets in their own right. -- Enceladus, Titan and Triton for example, also show the range of bodily differences between the bodies.

Bob Ware: 05/12/2014 11:11 CDT

Jaime - Thanks for the article and science behind the data.

Olivier De Goursac: 07/10/2014 01:51 CDT

Yes Jaime - Thanks also so much for all those good science articles of yours ! :=) :=) :=)